Glow cathode



Sept 5, 1939- H. KoLLlGs ET AL 2,172,207

GLOW CATHODE Filed July 5l, 1937 Patented Sept. 5, 1939 UNITED STATESv PATENT FFICI?.

ter, Berlin, Germany,

assignors .to Siemens &

Halske, Aktiengesellschaft, Siemensstadt, near Berlin, Germany,

a corporation of Germany Application July 31, 1937, Serial No. 156,764 In Germany September 19, 1936 1Claim.

The invention relates to cathodes, and especially to cathodes for heavy duty.

An object of the invention is to provide a hot cathode structure in which the coating is firmly secured to its base and in which there is a good conducting contact between the electron emitting coating and its base.

Other objects and advantages of the invention will be apparent from the following description and drawing in which the single figure is a crosssection of a preferred embodiment of the invention.

The invention especially relates to heavy duty, hot cathodes for gas or vapor filled discharge devices, although it could be utilized also for high vacuum discharge devices. According to the invention, the cathode has a metal base which serves as a carrier for the electron emissive material of alkali or earth alkali metals and compounds, or mixtures of these metals. The surface of the base is sintered with small4 metal particles which provide a very large number of small cracks and openings providing irregular projections on the surface of the base. 'Ihe electron emissive coating is held rigidly in these cracks and openings. The metal particles that are sintered onto the base provide a good heat conductive connection thereto, so that local overheating of the emissive material shall not take place and result in local ignition of the electron emissive layer.

A tube such as the one illustrated in the drawing is preferred as the carrier for the metal particles sintered thereto. This tube, which may ,be hollow or solid, is preferably of nickel or copper or an alloy of these metals and the heating current is preferably conducted through this tube. The tube is utilized by reason of the comparatively large surface for the coating, and

cathodes of the same dimensions may be provided for diierent heating currents and potentials by varying the wall thickness. Alloys having a high heat resistivity quality may also be utilized for the tube or other shaped base. In this connection alloys which in addition to iron or nickel contain admixtures or chronium, manganese, beryllium, tungsten or molybdenum are especially desired. Because the sintered metallic layer can cover the surface of the tube, materials can be used for the tube that could not be otherwise utilized in view of a danger of reaction with the electron emissive layer, which reaction would result in a decrease in the activity of the cathode.

The metal particles are preferably irst de- (Cl. Z50-27.5)

posited on the metal tube or other base with the aid of a suitable binding material. The metal particles may be shavings or powder or dust in accordance with the size desired. 'I'hese metal particles may be made into a strokable or pliable 5 mixing the particles with par- 'Ihe mass may also contain a mixture of particles of different size as, for example, of shavings and powder.

After the mass is put on the tube or other base, 10 the binding material is removed by heating and then the deposited metal layer is sintered to the base. It is desirable to carry out the sintering by electric heating of the base which may be done in the form disclosed in the drawing by 15 'conducting an electrical current through the tube base.

It is preferred to maintain the ambient region comparatively coo l so that there is a large temperature dierence between the surface of the 20 carrying base and the outer portions of the metal particles. Themetal particles will partially melt at least on their contact surface with the base and be sintered or fused onto the base. By maintaining the ambient region comparatively 25 cool, the outer surface of the metal particles will maintain their rough form. 'I'he sintering preferably takes place in a protective gas atmosphere or in a vacuum. It may be desirable to utilize small amounts of gas such as oxygen.

The deposit of small metal particles may also take place by putting on the base a carrier and then imbedding the particles of metal in the carrier and then raise it to a sufficiently high temperature. The carrier will then be removed by 35 the heat and the metal particles will adhere to the base.

The metal particles preferably consist of nickel, copper, silver or iron, or alloys of these metals with each other or with other materials. 'I'he 40 materials should be easily convertible into a suitable grainy or powdery form. Alloys of metal such as chromium alloys which have a considerin question chemically or by separating it from vapor forming compounds such as carbonyl.

If nickel is utilized as the base and nickel particles as the coating layer, it is preferable to 50 raise the temperature gradually to 1000 C. during the sintering process and to maintain the temperature at this value approximately iive to ten minutes.

The electron emitting material is then deposited 55 the base. pounds of alkali or earth alkali metals suchas the oxides or carbonates o! these metals and also mixtures of these compounds. The electron emitting material may be applied by spraying. dipping, distilling or stroking the material on in one or more thin layers. These compounds may be converted into oxides by heating the cathode. The cathode may be activated with the ald of an electrical discharge or also by vaporlaing active metal such as barium in combination with a simultaneous or subsequent conduction of an electric discharge. Aluminum may be added to the material for reducing purposes. Under certain circumstances, such a reducing material or materials may be.added to the layer deposited by sintering or by using alloys for such sintering which contain these materials. When the active material, for example, barium or strontium, is vaporized onto a cathode, the deposit of an alkali or earth alkali metal compound containing layer may be omitted, especially when oxidizable metals, for example, copper or silver in finely divided form are sintered onto the base or carrier, especially where the base is a nickel or tungsten wire or tube.

In the above described methods the electron emissive material is held fast in the cracks and openings formed by the sintered metal particles. The assembly can be more surely secured to the base by depositing alternately layers of metal particles and emissive materials and either sintered on each individual metal layer or al1 the layers together. The layers consisting of high electron emissivity material such as alkali or earth alkali metals are preferably comparatively thin, so that the metal particles will have contact with each other over a large part of the surface and be rigidly sintered to each other. This type of assembly provides a sort oi cage composed of very small metal particles in good electrical and heat conductive connection with the base and containing the active electron emitting material. This type of assembly prevents any `loosening of the emissive materials and when such a cathode is utilized in rectiiler `tubes, screens which cover the cathode from the anode may be dispensed with. Although the additional metal particles sintered on by the above described methods were in the form of chips, dust or powder, it is also possible to deposit metal in sufficiently iinely divided form electrolytically. Suillciently high current ,densities should be utilized so that the metal being thus disposed is precipitated in spongy form and then sintered as described above. The absorbed gases will be driven oi! at the time of sintering and thentheemisaivelayerplacedtheroon.

The cathodes may be produced metal base covered by a sintered metal layer continuous lengths o! a long wire or tube with a finely divided metal containing layer sprayed thereon. The binding Dmaterial may be driven oi! or tube may then be cut to lengthaiter it has left the iurnacevor after it is cooled in a protective gas atmosphere.

Other types of cathodes may be formed according to the cathode such as the indirectly heated type of cathode, Anothertype is that in which a hollow body is provided with projections or ribs to increase the surface which carries the emissive layer. 'I'hese surfaces may be treated in the above described manner. The inside of the cathode may be iilled with a suitable powder or with chips such as nickel chips and heated with the aid of high frequency current. On the surface of the cathode a large number of metal particles are then sintered. The excessive portion of the nlled chips may then be removed. A cathode so treated may be provided with an active coating in any manner described above. 'Ihe coating may be heated internally or externally indirectly. The heating body for the cathode may be covered with an active layer in the above described manner. An indirectly heated cathode .or hollow cathode may also be produced by winding a wire covered with metal particles sintered thereon on a mandrel. 'Ihis mandrel may be an insulating body which contains a suitable heating wire. The mandrel may be removed, however, after the wire is wound and the resulting cylindrical structure may be utilized as the emission body of a hollow cathode. The actual emission layer may be deposited before or after the winding of the wire as desired.

While there has been described certain embodiments and methods ot the invention, it is apparent that many changes may be made therein without departing from the spirit ot the invention. Accordingly, it is desired that o nly such limitations be imposed upon the following claim as is necessitated by the prior art.

We claim as our invention:

A cathode comprising a hollow metal tube, a layer of metal particles selected from the group consisting of nickel, copper, silver and iron, said layer having a sinteredcontact with said tube and an electron emitting coating on said metal particles, said coating containing a metal selected from the alkali metal and alkali-earth metal groups.

HANS KOLLIGB. 

